4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



Types. — Holotype U.S.N.M. No. 124193a; figured paratypes 

 U.S.N.M. Nos. I24i86a-e ; measured paratype U.S.N.M. No. 124193b. 



Horizon and locality. — Mural limestone, from a small hill 300 yards 

 east of U. S. Highway 80, NWiSW^NEi sec. 36, T. 23 S., R. 25 E., 

 Bisbee Quadrangle, Arizona, 



Discussion. — This species is characterized by its moderate size, con- 

 vex valves and costellate surface. One of the significant features of 

 the Arizona species is the poor development of the fold and sulcus, 

 which is strongly developed only at the front. A few specimens of 

 rhynchonellids have been found in the American Lower Cretaceous 

 which are probably referable to Cyclothyris. None of these resembles 

 the present species. The National Museum has a specimen of Cyclo- 

 thyris from the Main Street formation, Fort Worth, Tex., but it is 

 transversely elliptical rather than strongly triangular as the Mural 

 species is. Of three specimens of Cyclothyris from the Edwards lime- 

 stone, Presidio County, Texas, two are strongly transverse, but the 

 third is suggestive of C. amcricana. 



The British species most suggestive of C. americana is C. nuciformis 

 (Sowerby), but this differs in having a pronounced fold and sulcus 

 for at least half the valve length. Some young specimens from the 

 Cretaceous of France called Cyclothyris compressa (Lamarck) or 

 C. depressa (Sowerby) suggest the American species but they are 

 usually more transverse or are demonstrable aberrations of a variable 

 species. 



CYCLOTHYRIS species 



A single specimen (U.S.N.M. No. 124216) differing importantly 

 from C. americana was found with the other specimens described 

 herein. Although somewhat crushed, it differs from C. americana in 

 being much wider and in having a fairly prominent fold and sulcus 

 that originate a short distance anterior to the middle. 



RECTITHYRIS VESPERTINA Cooper, new species 



Plate iB, figures 18-37 



Shell small for the genus, elongate oval in outline and with the 

 maximum width at the middle ; sides gently rounded ; anterior margin 

 narrowly rounded. Valves unequal in depth, the pedicle valve having 

 the greater depth. Posterior margin narrowly rounded to subcarinate ; 

 anterior commissure rectimarginate to faintly uniplicate ; lateral com- 

 missure nearly straight. Surface smooth except for concentric lines 

 and varices of growth. 



